The degree of combustion of an engine depends on the compression ratio of the fuel mixture in the engine chamber. A highly compressed fuel mixture makes relatively perfect combustion and provides higher efficiency. But in practice, when the compression ratio exceeds 10 for a gasoline engine or 22 for a diesel engine, the mixture will burn and thereby create power to resist the inward travel of the engine piston. Thus a higher compression ratio of fuel mixture in an engine is difficult to acquire.
Moreover, the resistance to piston travel before its returning point (the maximum inward position of the piston) not only causes power losses but also shocks the engine itself.
A low fuel/air mixture might permit a higher compression ratio without burning itself, but it is another problem whether the engine structure can withstand the pressure.